Pulverulent material has been subjected to reduction of particle size in fluid energy mills for many years, but the expense of such treatment has rendered it impractical for all except certain limited applications.
Fluid energy mills rely on the introduction of particulate material into a vessel having a high-velocity, normally sonic or supersonic velocity, fluid medium recirculating therein. The circulating flow of fluid medium is normally used to effect a centrifugal separation of the particulate material to permit a withdrawal of the finely ground material while the coarse material continues its recirculation. The coarse material is reduced in size either by impingement against other particles in the recirculating flow or else by impingement against the vessel walls. In the former case, there is considerable loss of energy in the prior art ways of causing the inter-particle impingement, and in the latter case, there is substantial erosion of the vessel walls due to the high speed impact of the particles against the walls.
U.S. Pat. No. 4,219,164 reviews the then state of the art respecting fluid energy comminution and the problems associated with the practice thereof. That patent discloses a new and improved apparatus and method for fluid energy comminution comprising the injection of sonic velocity fluid through a plurality of nozzles located around the base of a cylindrical vessel directed inwardly and offset from a radius so as to provide a comminution zone and simultaneously induce a vortex which traverses upwardly the central portion of the vessel to provide a centrifugal field for the classification of the particulates in the flow. An axially disposed outlet at the top of the vessel allows the product stream, from which oversized particulate has been centrifuged, to exhaust, the remaining flow containing oversize particulate material entrained in carrier medium being recirculated outward and downward around the outer portion of the vessel to be entrained in the sonic flow issuing from the nozzles so as to again be subjected to comminution. New feed material is continually added to the system. Although this apparatus has been demonstrated to effectively produce large quantities of pulverulent product (over 20 tons per hour) difficulties have been experienced in maintaining proper nozzle alignment when using superheated steam as the carrier medium due to thermal distortion. Improper nozzle alignment causes distortions in the vortex and degrades classification. Furthermore, spurious secondary circulation flows have from time to time been found to interfere with proper operation. Lastly, the inward flow of sonic velocity carrier medium limits the throughput capacity, which may be less than that desired.